Secure Data Transmission Using Spatial Multiplexing

ABSTRACT

An example transmitter includes an encoder, a plurality of modulators and a spatial multiplexer. The encoder is configured to encode one or more input bit streams into a plurality of coded bit streams which are provided as output. Each modulator is configured to receive and modulate a respective one of the plurality of coded bit streams and to provide a modulated output signal. The spatial multiplexer is configured to spatially multiplex the plurality of modulated output signals for insertion on a spatially multiplexing waveguide. A modulated output signal(s) may be inserted onto a mode of a multi-mode fiber or onto a core of a multi-core fiber. In one embodiment, a subset of the plurality of spatially multiplexed modulated output signals which correspond to the one or more input bit streams must be simultaneously and spatially selectively detected in order to recover a first input bit stream.

PRIORITY CLAIM

The application claims benefit of U.S. Provisional Application No.61/431,040 filed, Jan. 9, 2011.

CROSS REFERENCE TO RELATED APPLICATIONS

The subject matter of this application is related to that of U.S. patentapplication Ser. No. ______, by Peter J. Winzer, attorney docketreference 809508, filed on the same date and the present application,and entitled “SECURE DATA TRANSMISSION USING SPATIAL MULTIPLEXING”,which application is incorporated herein by reference in it entirety.

FIELD OF THE INVENTION

The invention relates to optical transmission equipment and, morespecifically but not exclusively to the equipment that enables datatransmission using spatial multiplexing.

BACKGROUND INFORMATION

This section introduces aspects that may help facilitate a betterunderstanding of the disclosed invention(s). Accordingly, the statementsof this section are to be read in this light and are not to beunderstood as admissions about what is in the prior art or what is notin the prior art.

Today's fiber-optic optical transmission systems are either based onsingle-polarization or polarization-division multiplexedsingle-transverse-mode transmission, or on multi-mode transmission whereall modes essentially carry the same data. Both such systems arevulnerable to tapping, which is done by bending the transmission fibersuch that a small amount of light is coupled out at a local bend. Thetapped light is then detected by a sensitive receiver at the location ofthe fiber bend, giving an eavesdropper access to the fullwavelength-division multiplexed (WDM) spectrum transmitted over thefiber. The small amount of extra loss is not easily detectable at therightful receiver located at the end of the transmission fiber, whichpermits the eavesdropper to go unnoticed.

A method of detecting the presence of an eavesdropper in a single-modefiber system is furnished in U.S. Patent Application Publication No.2008/001888, by D. Butler et al, published Jan. 24, 2008, and entitled“Intrusion Detection in Optical Fiber Networks,” which application isincorporated herein by reference in its entirety, and whereintransmission is done at several optical wavelengths, and a change in thedifferential loss between these wavelength is used as an indicator forthe presence of an eavesdropper.

SUMMARY

The following presents a simplified summary of the disclosed subjectmatter in order to provide an understanding of some aspects of thedisclosed subject matter. This summary is not an exhaustive overview ofthe disclosed subject matter and is not intended to identify key orcritical elements of the disclosed subject matter not to delineate thescope of the disclosed subject matter. Its sole purpose is to presentsome concepts in a simplified form as a prelude to the more detaileddescription that is discussed later.

The systems described above in the Background of the Specification haveat least two important detriments. First, single-mode and conventionalmulti-mode fibers are inherently vulnerable to bend-induced tapping.Second, a system using in-line optical amplifiers will generallyequalize the power levels of the amplified signals at all wavelengthsand will hence convert a wavelength-dependent loss differential into asignal-to-noise ratio (SNR) differential, which in turn prohibits thedetection of an eavesdropper by a simple differential loss measurementat the receiver.

Furthermore, while quantum-based techniques to obtain physical-layersecurity may be fundamentally robust to eavesdropping, they are notcompatible with an optically amplified transmission infrastructure,which severely limits their reach as well as their applicability inrealistic commercial networks.

Embodiments provided herein address one or more of the detrimentsassociated with transmitting data using physical-layer security based onclassical optical communications without the need to resort toquantum-cryptographic techniques such as quantum key distribution. Oneor more embodiments herein disclosed allow for secure physical-layertransmission over spatially multiplexed (e.g., multi-core or multi-mode)optical fiber. One or more disclosed embodiments allow for reception ofsecure physical-layer transmissions over spatially multiplexed (e.g.,multi-core or multi-mode) optical fiber. Still other disclosedembodiments provide the ability to identify the presence of aneavesdropper tapping into such fiber utilized for such spatiallymultiplexed physical-layer transmission through the use ofspatially-resolved differential loss or Signal-To-Noise-Ratio (SNR)measurements.

In one embodiment, a transmitter includes and encoder, a plurality ofmodulator and a spatial multiplexer. The encoder is configured to encodeone or more input bit streams into a plurality of coded bit streams andprovide the plurality of coded bit streams as output. Each of theplurality of modulators is configured to receive a respective one of theplurality of coded bit streams, modulate the respective one of theplurality of coded bit streams, and provide a modulated output signal.The spatial multiplexer configured to spatially multiplex the pluralityof modulated output signals for insertion on a spatially multiplexingwaveguide

The spatial multiplexer may be configured to provide at least one of thespatially multiplexed plurality of modulated output signals to a mode ofa multi-mode fiber or a core of a multi-core fiber. The modulator mayperform modulation using a polarization-multiplexed complex-valuedoptical modulation format, an intensity-modulated optical modulationformat, on/off keying, polarization-multiplexed quadrature phase shiftkeying, or polarization-multiplexed quadrature amplitude modulation.Other modulation formats may be utilized.

In one embodiment, recovery of the one or more input bit streamsrequires that all of the plurality of spatially multiplexed modulatedoutput signals corresponding to the one or more input bit streams to bedetected must be simultaneously and spatially selectively detected. Inanother embodiment, a subset of the plurality of spatially multiplexedmodulated output signals which correspond to the one or more input bitstreams must be simultaneously and spatially selectively detected inorder to recover a first input bit stream.

In one embodiment, the encoder encodes information of the one or moreinput bit streams in parity of bits of the plurality of coded bitstreams, and the plurality of modulators are configured to modulate theplurality of coded bit streams for all modes or cores at a same transmitwavelength and within a same symbol time slot. In another embodiment,the plurality of modulators is configured to modulate the plurality ofcoded bit streams for each mode or core at a different transmitwavelength. In yet another embodiment, the plurality of modulators areconfigured to modulate the plurality of coded bit streams for modes orcores within a plurality of different symbol time slots

The encoder may encode information of the one or more input bit streamsin a logic combination or algebraic combination of information containedin bits of the plurality of coded bit streams. The logical combinationmay includes an “and”, “or”, or “xor” operation between coded bit streambits; the algebraic combination may include an algebraic sum,difference, or product between the coded bit stream bits.

In one embodiment, the transmitter includes a mode selective detectorconfigured to convert a plurality of modes received from the spatiallymultiplexing waveguide into a plurality of coded bit streams, whereinthe number of coded bit streams is less than or equal to the number ofmodes; and a decoder configured to perform a reverse operation theencoder, the decoder being configured to produce one or more output bitstreams from the plurality coded bit streams.

In one embodiment, a method for secure transmission includes encoding byan encoder one or more input bit streams across a plurality of coded bitstreams, modulating by a modulator respective ones of the plurality ofcoded bit streams to form a plurality of modulated signals, andspatially multiplexing by a spatial multiplexer the plurality ofmodulated output signals for insertion on a spatially multiplexingwaveguide. The method may also include inserting the plurality ofspatially multiplexed modulated signals into a spatially multiplexingwaveguide.

In other embodiment, the spatially multiplexing waveguide may be amulti-mode fiber or a multi-core fiber. The modulating of anotherembodiment may utilize a polarization-multiplexed complex-valued opticalmodulation format, an intensity-modulated optical modulation format,on/off keying, polarization-multiplexed quadrature phase shift keying orpolarization-multiplexed quadrature amplitude modulation or the like.

Recovery of the one or more input bit streams may require simultaneousand spatially selective detection of all of the plurality of spatiallymultiplexed modulated output signals corresponding to the one or moreinput bit streams to be detected in one embodiment. In anotherembodiment, a subset of the plurality of spatially multiplexed modulatedoutput signals that correspond to the one or more input bit streams mustbe simultaneously and spatially selectively detected in order to recovera first input bit stream.

In one embodiment, encoding encodes information of the one or more inputbit streams in parity of bits of the plurality of coded bit streams. Invarious embodiments, modulating may modulate the plurality of coded bitstreams for all modes or cores at a same transmit wavelength and withina same symbol time slot, may modulate the plurality of coded bit streamsfor each mode or core at a different transmit wavelength, or maymodulate the plurality of coded bit streams for modes or cores within aplurality of different symbol time slots. Modulation may includemodulating information of the one or more input bit streams in a logiccombination or algebraic combination of information contained in bits ofthe plurality of coded bit streams.

In an example embodiment, a receiver includes a mode selective detectorconfigured to convert a plurality of modes received from a spatiallymultiplexing fiber into a plurality of coded bit streams, wherein thenumber of coded bit streams is less than or equal to the number ofmodes, and a decoder configured to decode information for one or moreoutput bit streams from parity of the plurality of coded bit streamsbits, wherein the plurality of coded bit streams for all the modes areprovided at a same transmit wavelength and within a same symbol timeslot.

The plurality of modes may be received in a polarization-multiplexedcomplex-valued optical modulation format, an intensity-modulated opticalmodulation format, on/off keying format, a polarization-multiplexedquadrature phase shift keying format, or a polarization-multiplexedquadrature amplitude modulation format and like.

In one embodiment, recovery of the one or more output bit streamsrequires that all of the plurality of modes corresponding to the one ormore output bit streams be detected simultaneously and spatiallyselectively. In another embodiment, a subset of the plurality of modeswhich correspond to the one or more bit streams must be simultaneouslyand spatially selectively detected in order to recover a first input bitstream.

The decoder may be configured to decode information for the one or moreoutput bit streams from parity of bits of the plurality of coded bitstreams. The decoder may decode information for the one or more outputbit streams based on a logical combination including at least one of an“and”, “or”, or “xor” operation performed between coded bit stream bits.In another embodiment, the decoder is configured to decode informationfor the one or more output bit streams based on an algebraic combinationincluding at least one of an algebraic sum, difference, or productbetween coded bit stream bits.

In one embodiment, the receiver includes a measurement module formeasuring a parameter for the plurality of modes received from thespatially multiplexing fiber, wherein the parameter is a power or asignal to noise ratio (SNR), a difference calculator for comparing themeasured parameter among a subset modes and/or among a known set ofunperturbed parameters and determining a differential, the subsetincluding at least one mode, and a threshold and alarm module forsetting an alarm indicator when the differential is out of bounds. Thethreshold and alarm module may be configured to set the alarm indicatorwhen the differential changes by an amount greater than a threshold.

An example method includes receiving at a receiver a spatiallymultiplexed signal, measuring a parameter for each mode of the spatiallymultiplexed signal, wherein the parameter is a power or a signal tonoise ratio (SNR), comparing the measured parameter among a subset ofmodes and/or among a known set of unperturbed parameters and determininga differential, the subset including at least one mode, and setting analarm indicator when the differential is out of bounds. It may bedetermined that the differential is out of bounds when the differentialchanges by an amount greater than a threshold. The method may beperformed optically or electronically.

In one embodiment, the method also includes converting a plurality ofmodes received in the spatially multiplexing signal into a plurality ofcoded bit streams, wherein the number of coded bit streams is less thanor equal to the number of modes, and decoding information for one ormore output bit streams from parity of the plurality of coded bitstreams bits, wherein the plurality of coded bit streams for all themodes are provided at a same transmit wavelength and within a samesymbol time slot. In one embodiment, a subset of the plurality of modeswhich correspond to the one or more bit streams must be simultaneouslyand spatially selectively detected in order to recover a firsttransmitted bit stream.

One example apparatus comprises a mode selective detector for detectinga plurality of modes of a spatially multiplexed signal, a measurementmodule for measuring a parameter for the plurality of modes of thespatially multiplexed signal, wherein the parameter is a power or asignal to noise ratio (SNR), a difference calculator for comparing themeasured parameter among a subset modes and/or among a known set ofunperturbed parameters and determining a differential, the subsetincluding at least one mode, and a threshold and alarm module forsetting an alarm indicator when the differential is out of bounds. Atleast one of the mode selective detector, the measurement module, thedifference calculator and the threshold and alarm module may be opticalelements. At least one of the mode selective detector, the measurementmodule, the difference calculator and the threshold and alarm module areelectronic elements.

In one embodiment, the apparatus also includes at least one of the modeselective detector which is configured to decode information for one ormore output bit streams from parity of the plurality of coded bitstreams bits, wherein the plurality of coded bit streams for all themodes are provided at a same transmit wavelength and within a samesymbol time slot.

BRIEF DESCRIPTION OF THE DRAWINGS

The example embodiments will become more fully understood from thedetailed description given herein below and the accompanying drawings,wherein like elements are represented by like reference numerals, whichare given by way of illustration only and thus are not limiting of theexample embodiments and wherein:

FIG. 1 illustrates an example system for secure data transmissionincluding example transmitter, optical link and receiver;

FIG. 2 illustrates a RX-side detection mechanism to determine thepresence of a fiber tapping eavesdropper.

DETAILED DESCRIPTION

Various example embodiments will now be described more fully withreference to the accompanying figures, it being noted that specificstructural and functional details disclosed herein are merelyrepresentative for purposes of describing example embodiments. Exampleembodiments may be embodied in many alternate forms and should not beconstrued as limited to only the embodiments set forth herein.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms since such terms are only used to distinguishone element from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments.Moreover, a first element and second element may be implemented by asingle element able to provide the necessary functionality of separatefirst and second elements.

As used herein the description, the term “and” is used in both theconjunctive and disjunctive sense and includes any and all combinationsof one or more of the associated listed items. It will be furtherunderstood that the terms “comprises”, “comprising,”, “includes” and“including”, when used herein, specify the presence of stated features,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which example embodiments belong. Itshould also be noted that in some alternative implementations, thefunctions/acts noted may occur out of the order noted in the figures.For example, two figures shown in succession may in fact be executedsubstantially concurrently or may sometimes be executed in the reverseorder, depending upon the functionality/acts involved.

Spatial multiplexing in multi-core or multi-mode optical waveguides isutilized for physical-layer data security according to the disclosedembodiments of the inventions. FIG. 1 illustrates an example system 100for secure data transmission including example transmitter 110, opticallink 130 and receiver 150. As shown in FIG. 1, one or more secure bitstreams 102 are encoded by a transmitter 110 into M_(TX)≦M modes orcores 104 supported by a spatially multiplexing waveguide 130 in such away that one needs to simultaneously and/or spatially selectively detectall (or a subset of) the modes/cores in order to access the transmittedinformation at the receiver 150. As mentioned, the spatiallymultiplexing waveguide 130 may be a multi-mode or multi-core fiber.

While the step of encoding by an encoder 106 at the transmitter 110 isdescribed in more detail below, the step of modulation by a modulator108 at the transmitter 110 uses intensity-modulated optical modulationformats (such as on/off keying), or more generallypolarization-multiplexed complex-valued optical modulation formats (suchas polarization-multiplexed quadrature phase shift keying or quadratureamplitude modulation and the like).

Regarding the step of mode shaping by a mode shaper 111 at thetransmitter 110, methods for transmitting and receiving information in amode-selective way are disclosed in U.S. Patent Application PublicationNo. 2010/0329670, by R. Essiambre et al, published Dec. 30, 2010, andentitled “Receiver for Optical Transverse-Mode-Multiplexed Signals,” andU.S. Patent Application Publication No. 2010/0329671, by R. Essiambre etal, published Dec. 30, 2010, and entitled “Transverse-Mode-MultiplexingFor Optical Communication Systems,” and both applications areincorporated herein by reference in their entirety. In particular, thepossibility of performing polarization-multiplexed WDM transmission oneach mode is contemplated in one embodiment of the present inventioneven though this is not shown in the figure for the reason of theclarity of the illustration.

In FIG. 1, the illustrated transmitter 110 includes an encoder 106, aplurality of modulators 108 and a mode shaper 112. The encoder 106encodes one or more input bit streams 102 into a plurality of coded bitstreams 114 and provides the plurality of coded bit streams as output ofthe encoder. Each of the plurality of modulators 108 receives arespective one of the plurality of coded bit streams 114, modulates therespective one of the plurality of coded bit streams, and provides amodulated output signal 116. The mode shaper 112 spatially multiplexesthe plurality of modulated output signals 116 for insertion on aspatially multiplexing waveguide 130.

“M_(TX)”, “M”, and “M_(RX)”, i.e., the number of modes the transmitterlaunches 104, the number of modes supported by the waveguide (notexplicitly shown), and the number of modes extracted 156 by the receiver150 may be different. Obviously, M_(TX), M_(RX) may be ≦M. Thedifference between these number of mode launched and extracted isindicated by the varying numbers of arrows at the input and output ofthe fiber illustrated in FIG. 1.

Regarding the step of encoding, many spatially spreading codes could beenvisioned in this context. One possible code encodes an input bitstream in the parity of the bits transmitted on all or a subset ofmodes/cores at the same transmit wavelength and within the same symboltime slot. Unless all modes/cores across which the input bit stream isbeing spread are correctly and simultaneously detected, the parity ofthe resulting information bits cannot be determined and hence the inputbit stream remains secure. Another code encodes the input bit stream inthe parity of the bits transmitted on all or a subset of modes/cores atdifferent transmit wavelengths and/or within different symbol timeslots, taking advantage of time-varying and wavelength-dependent modecoupling that occurs during propagation. For example, mode 1 may alwaysbe launched at lambda1, mode 2 at lambda2, mode 3 on lambda3, etc. Forexample, mode 2 may be launched, say, 3 time slots away from mode 1,with mode 3 being launched 40 time slots away from mode 1, etc.

Instead of using “parity” to encode information of the input bit streamyet another code encodes the information in a logic or algebraiccombination of the information contained in each employed mode, such ascombinations of logical “and”, “or”, or “xor” combinations between modalbit streams, or algebraic sums, differences, products, etc.Alternatively, each input bit stream may be directly launched into oneof the M modes (or a subset of modes) while one or more of the othermodes are being used to transmit other signals (e.g., random orinformation bearing signals; e.g., redundancy for error-correctionschemes) that would act as interferes upon a fiber tapping attack. Theapplicability of different secure coding schemes that may be utilized incombination with spatially multiplexed transmission will be apparent toa person skilled in the art. Note, as stated above, in one embodimentthe encoder 106 acts as a pass though device for a bit stream 102 (i.e.,the encoder essentially does nothing to modify the format of an inputbit stream; that is, does not modify the content of an input bit streambut merely provides the input bit stream in the same form as a one ormore coded bit streams (at least in name as described herein) that areoutput from the encoder) and where the security provided by suchembodiments of a transmitter and communication system rely on the factthat multiple bit streams are present in the fiber during transmission.

In a spatially multiplexed multi-core or multi-mode optical fiber,waveguide bending will lead to light leakage out of the fiber, inanalogy to single-mode or conventional multi-mode fiber. However, thespatial degree of information will be severely degraded or evencompletely lost during the fiber tapping process. Hence, since afiber-tapping eavesdropper will no longer be able to correctly decodethe information transmitted on individual modes, the secure informationwill be inevitably rendered useless to the fiber tapping eavesdropper.

In FIG. 1, the illustrated example receiver 150 includes a modeselective detector 152 and a decoder 154. In one embodiment, the modeselective detector 152 is configured to convert M_(RX) modes received156 from the spatially multiplexing fiber 130 into a plurality of Kcoded bit streams 158 (wherein K is less than or equal to M, M being anumber of modes supported by a waveguide) and the decoder 154 configuredto provide the reverse operation of any of the encoder at thecorresponding transmitter and to produce one or mode output bit streams160 from the K coded bit streams. In another embodiment, the modeselective detector 152 is configured to convert a plurality of modesreceived from the spatially multiplexing fiber (i.e., a plurality ofreceived modes 156) into a plurality of coded bit streams 158, whereinthe number of coded bit streams is less than or equal to the number ofmodes, and the decoder 154 is configured to decode information for oneor more output bit streams 160 from parity of the plurality of coded bitstreams bits, the plurality of coded bit streams corresponding to asignal to be recovered and being provided at a same transmit wavelengthand within a same symbol time slot for all modes/cores.

FIG. 2 illustrates a RX-side detection mechanism to determine thepresence of a fiber tapping eavesdropper. As an additional aspect, andas shown in FIG. 2, the presence of an eavesdropper is detectable at alegitimate communication receiver 200 by measuring the loss differentialcompared to un-perturbed operation (or, in a system using opticalamplification, the SNR differential compared to un-perturbed operation)between received modes. Both quantities can be made available to thereceiver, e.g., through mode-deconvolving multiple-input multiple-output(MIMO) digital signal processing algorithms that are an integral part ofsome spatially multiplexing receivers, or through optical power or SNRmeasurements after optical mode separation (e.g., in the case ofmulti-core fiber with essentially uncoupled cores, for which the MIMOsignal processing would not necessarily be implemented at the receiver).Accordingly, an eavesdropper detecting apparatus according to theprinciples disclosed here may be implemented as a stand-alone device 200or as part of a receiver 200.

Whenever the measured power or SNR differentials deviate by anadjustable uncertainty margin from their normal (untapped) values, thereceiver concludes the presence of a fiber bend and triggers an alarm toalert the operator of the potential presence of an eavesdropper.Depending on the system configuration, differential power or SNRmeasurements can also be based on any hybrid combination of K spatialmodes and L wavelengths.

FIG. 2 illustrates an example apparatus for determining the presence ofa fiber tapping eavesdropper. As illustrated, the apparatus 200 includesa mode selective detector 210 for detecting a plurality of modes 212 ofa spatially multiplexed signal, a measurement device 214 for measuring aparameter for the plurality of modes of the spatially multiplexedsignal, wherein the parameter is a power or a signal to noise ratio(SNR), a difference calculator 216 for comparing the measured parameteramong a subset modes and/or among a known set of unperturbed parametersand determining a differential, the subset including at least one mode,and a threshold and alarm device for setting an alarm indicator 220 whenthe differential is out of bounds. At least one of the mode selectivedetector, the measurement module, the difference calculator and thethreshold and alarm module may be optical elements. At least one of themode selective detector, the measurement module, the differencecalculator and the threshold and alarm module are electronic elements.

For example, a MIMO DSP as disclosed in the Essiambre et al patentsapplication cited above may be utilized in one embodiment. A by-productof the MIMO DSP algorithms can be differential loss and differentialSNR. Thus, the steps of the proposed methodology may be accomplishedall-electronically. In case of essentially uncoupled multi-core, onewould not use MIMO DSP. Thus, in one embodiment, the receiver 160 ofFIG. 1 can optically tap off a fraction of the light received from eachcore (tapping not shown). The tapped-off light can then be analyzedoptically, e.g., by optical power measurement, wavelength-resolvedoptical SNR measurement, etc. (These monitoring techniques themselvesare known to one skilled in the art.) Once power or SNR are measured,the remaining steps of the methodology may be accomplishedelectronically (parameter comparison, threshold, alarm). Of course,hybrid solutions are possible.

In one embodiment, a method includes receiving a spatially multiplexedsignal; measuring a parameter for each mode of the spatially multiplexedsignal, wherein the parameter is a power or a signal to noise ratio(SNR); comparing the measured parameter among a subset of modes and/oramong a known set of unperturbed parameters and determining a set ofdifferentials, the subset of modes including at least one mode; andsetting an alarm indicator when the set of differential indicates an outof bounds condition.

There are three aspects for the above comparing of parameters anddetermining of differentials. In one embodiment, differentials may becompared among measured parameters only. In another embodiment, thedifferentials are determined based on comparison made between measuredparameters and corresponding nominal (un-perturbed) value, for example,a nominal value stored in a look-up table. In yet another embodiment,the comparison utilizes of combination of both of the above comparisonmethodologies.

The set of differentials may be determined to be out of bounds when asum of the differentials or an individual differential changes by anamount greater than a threshold. The threshold may be a user-definedsecurity threshold such that a hyper-sensitive user can set thethreshold very tight and accept occasional false alarms, whereas alesser sensitive user may set the threshold looser and avoid falsealarms.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense to only the illustrated embodiments.

Embodiments of present invention may be implemented as circuit-basedprocesses, including possible implementation on a single integratedcircuit.

Unless explicitly stated otherwise, each numerical value and rangeshould be interpreted as being approximate as if the word“approximately” preceded the value of the value or range.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain the nature of this invention may be madeby those skilled in the art without departing from the scope of theinvention as expressed in the following claims.

The use of figure numbers and/or figure reference labels in the claimsis intended to identify one or more possible embodiments of the claimedsubject matter in order to facilitate the interpretation of the claims.Such use is not to be construed as necessarily limiting the scope ofthose claims to the embodiments shown in the corresponding figures.

Although the following method claims, if any, recite steps in aparticular sequence with corresponding labeling, unless the claimrecitations otherwise imply a particular sequence for implementing someor all of those steps, those steps are not necessarily intended to belimited to being implemented in that particular sequence.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of theinvention. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments necessarilymutually exclusive of other embodiments. The same applies to the term“implementation.”

Also for purposes of this description, the terms “couple,” “coupling,”“coupled,” “connect,” “connecting,” or “connected” refer to any mannerknown in the art or later developed in which energy is allowed to betransferred between two or more elements, and the interposition of oneor more additional elements is contemplated, although not required.Conversely, the terms “directly coupled,” “directly connected,” etc.,imply the absence of such additional elements.

The embodiments covered by the claims are limited to embodiments that(1) are enabled by this specification and (2) correspond to statutorysubject matter. Non-enabled embodiments and embodiments that correspondto non-statutory subject matter are explicitly disclaimed even if theyformally fall within the scope of the claims.

The description and drawings merely illustrate principles of theinvention. It will thus be appreciated that those of ordinary skill inthe art will be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope. Furthermore, allexamples recited herein are principally intended expressly to be onlyfor pedagogical purposes to aid the reader in understanding theprinciples of the invention and the concepts contributed by theinventor/s to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass equivalents thereof.

The functions of the various elements shown in the figures, includingany functional blocks labeled as “processors”, “controllers”, “devices”or “modules” may be provided through the use of dedicated hardware aswell as hardware capable of executing software in association withappropriate software. When provided by a processor, the functions may beprovided by a single dedicated processor, by a single shared processor,or by a plurality of individual processors, some of which may be shared.Moreover, explicit use of the term “processor” or “controller” or“module” should not be construed to refer exclusively to hardwarecapable of executing software, and may implicitly include, withoutlimitation, digital signal processor (DSP) hardware, applicationspecific integrated circuit (ASIC), field programmable gate array(FPGA), read only memory (ROM) for storing software, random accessmemory (RAM), and non-volatile storage. Other hardware, conventionaland/or custom, may also be included. Similarly, any switches shown inthe figures are conceptual only. Their function may be carried outthrough the operation of program logic, through dedicated logic, throughthe interaction of program control and dedicated logic, or evenmanually, the particular technique being selectable by the implementeras more specifically understood from the context.

It should be appreciated by those of ordinary skill in the art that anyblock diagrams herein represent conceptual views of illustrativecircuitry embodying the principles of the invention. Similarly, it willbe appreciated that any flow charts, flow diagrams, state transitiondiagrams, pseudo code, and the like represent various processes whichmay be substantially represented in computer readable medium and soexecuted by a computer or processor, whether or not such computer orprocessor is explicitly shown.

1. A transmitter comprising: an encoder configured to encode one or moreinput bit streams into a plurality of coded bit streams and provide theplurality of coded bit streams as output; a plurality of modulators,each modulator configured to receive a respective one of the pluralityof coded bit streams, modulate the respective one of the plurality ofcoded bit streams, and provide a modulated output signal; and a spatialmultiplexer configured to spatially multiplex the plurality of modulatedoutput signals for insertion on a spatially multiplexing waveguide. 2.The transmitter of claim 1 wherein the spatial multiplexer is configuredto provide at least one of the spatially multiplexed plurality ofmodulated output signals to a mode of a multi-mode fiber.
 3. Thetransmitter of claim 1 wherein the spatial multiplexer is configured toprovide at least one of the spatially multiplexed plurality of modulatedoutput signals to a core of a multi-core fiber.
 4. The transmitter ofclaim 1 wherein the modulator performs modulation using apolarization-multiplexed complex-valued optical modulation format, anintensity-modulated optical modulation format, on/off keying,polarization-multiplexed quadrature phase shift keying, orpolarization-multiplexed quadrature amplitude modulation.
 5. Thetransmitter of claim 1 wherein recovery of the one or more input bitstreams requires that all of the plurality of spatially multiplexedmodulated output signals corresponding to the one or more input bitstreams to be detected must be simultaneously and spatially selectivelydetected.
 6. The transmitter of claim 1 wherein a subset of theplurality of spatially multiplexed modulated output signals whichcorrespond to the one or more input bit streams must be simultaneouslyand spatially selectively detected in order to recover a first input bitstream.
 7. The transmitter of claim 1 wherein the encoder encodesinformation of the one or more input bit streams in parity of bits ofthe plurality of coded bit streams, and wherein the plurality ofmodulators are configured to modulate the plurality of coded bit streamsfor all modes or cores at a same transmit wavelength and within a samesymbol time slot.
 8. The transmitter of claim 1 wherein the encoderencodes information of the one or more input bit streams in parity ofbits of the plurality of coded bit streams, and wherein the plurality ofmodulators are configured to modulate the plurality of coded bit streamsfor each mode or core at a different transmit wavelength.
 9. Thetransmitter of claim 1 wherein the encoder encodes information of theone or more input bit streams in parity of bits of the plurality ofcoded bit streams, and wherein the plurality of modulators areconfigured to modulate the plurality of coded bit streams for modes orcores within a plurality of different symbol time slots.
 10. Thetransmitter of claim 1 wherein the encoder encodes information of theone or more input bit streams in a logic combination or algebraiccombination of information contained in bits of the plurality of codedbit streams.
 11. The transmitter of claim 10 wherein the logicalcombination includes an “and”, “or”, or “xor” operation between codedbitstream bits and wherein the algebraic combination includes analgebraic sum, difference, or product between the coded bitstream bits.12. The transmitter of claim 1, further comprising: a mode selectivedetector configured to convert a plurality of modes received from thespatially multiplexing waveguide into a plurality of coded bit streams,wherein the number of coded bit streams is less than or equal to thenumber of modes; and a decoder configured to perform a reverse operationthe encoder, the decoder being configured to produce one or more outputbit streams from the plurality coded bit streams.
 13. A method forsecure transmission, the method comprising: encoding by an encoder oneor more input bit streams across a plurality of coded bit streams;modulating by a modulator respective ones of the plurality of coded bitstreams to form a plurality of modulated signals; and spatiallymultiplexing by a spatial multiplexer the plurality of modulated outputsignals for insertion on a spatially multiplexing waveguide.
 14. Themethod of claim 13 further comprising: inserting the plurality ofspatially multiplexed modulated signals into a spatially multiplexingwaveguide.
 15. The method of claim 13 wherein the spatially multiplexingwaveguide is a multi-mode fiber or a multi-core fiber.
 16. The method ofclaim 13 wherein said modulating is performed using apolarization-multiplexed complex-valued optical modulation format, anintensity-modulated optical modulation format, on/off keying,polarization-multiplexed quadrature phase shift keying orpolarization-multiplexed quadrature amplitude modulation.
 17. The methodof claim 13 wherein recovery of the one or more input bit streamsrequires that all of the plurality of spatially multiplexed modulatedoutput signals corresponding to the one or more input bit streams to bedetected must be simultaneously and spatially selectively detected. 18.The method of claim 13 wherein a subset of the plurality of spatiallymultiplexed modulated output signals which correspond to the one or moreinput bit streams must be simultaneously and spatially selectivelydetected in order to recover a first input bit stream.
 19. The method ofclaim 13 wherein said encoding encodes information of the one or moreinput bit streams in parity of bits of the plurality of coded bitstreams, and wherein said modulating modulates the plurality of codedbit streams for all modes or cores at a same transmit wavelength andwithin a same symbol time slot.
 20. The method of claim 13 wherein saidencoding encodes information of the one or more input bit streams inparity of bits of the plurality of coded bit streams, and wherein saidmodulating modulates the plurality of coded bit streams for each mode orcore at a different transmit wavelength.
 21. The method of claim 13wherein said encoding encodes information of the one or more input bitstreams in parity of bits of the plurality of coded bit streams, andwherein said modulating modulates the plurality of coded bit streams formodes or cores within a plurality of different symbol time slots. 22.The method of claim 13 wherein said modulating modulates information ofthe one or more input bit streams in a logic combination or algebraiccombination of information contained in bits of the plurality of codedbit streams.
 23. The method of claim 22 wherein the logical combinationincludes an “and”, “or”, or “xor” operation between coded bitstream bitsand wherein the algebraic combination includes an algebraic sum,difference, or product between the coded bitstream bits.